This invention relates to improving the level of noise generated by a tire's tread as it travels upon a road surface, after the tread has been evaluated by the invention claimed in the parent application, and the tread has been deemed to generate an acceptable level of objectionable noise.
More specifically, this invention relates to simulating the effect on the level of noise associated with a patterned tread on a tire when it is operated on a road surface at any desired rotational speed, after making adjustments in the relative position of one rib with respect to another in the tread pattern. Thus, it will be appreciated that, though the method of this invention is applicable to any tread pattern having plural ribs, irrespective of the level of noise the tread generates when used on a wheel on an operating vehicle, it is most aptly directed to one which has been deemed acceptably quiet, but which promises to be even more so if the proper relative circumferential displacement of each rib relative to another can be derived by an appropriate simulation, that is, without physically producing a single tread. By circumferential displacement of one rib relative to another on the tread's surface, without changing the parallel relationship of the ribs in spaced-apart planes which are orthogonal to the longitudinal axis of the tire. Such circumferential displacement results in a rotational displacement of one rib relative to another.
A tire manufacturer seeks to design a tire tread which not only fulfills its function relating to wear, wet and dry traction, rolling resistance, and other desirable properties, but also runs with an acceptably low noise level. Preferably the tire should run at an acceptable noise level through the entire operating range of a particular vehicle on which the tire is to be mounted.
Since noise is inherently subjective, due to the psychoacoustical effects involved, there is no known mathematical criteria to define noise. It is therefore necessary to subjectively evaluate a tire for noise acceptability. Historically, this has been done by hand cutting a set of tires or building a mold to produce a set of tires for evaluation. This process is expensive and time consuming, thus limiting the number of trials available to the design engineer to optimize the design.
Much energy has been devoted to designing a tire tread which will spread the noise spectrum generated by the running tire. For example, U.S. Pat. Nos. 4,327,792 and 4,474,223 teach how to spread the noise over a wide range of frequency to reduce the tonality and undesirability of the sound. John H. Varterasian described a Mechanical Frequency Modulation method in SAE 690520, Quieting Noise Mathematically--Its Application to Snow Tires, 1969. The problem is that, no matter what criteria are used to design the tread, there is no assurance that the result will be a tread with an acceptable noise level. Whether the noise is spread over a wide range of frequency, or dominated by one or a few frequencies, is of secondary importance if the noise level is unacceptable.
There is presently in use a tire-noise simulator at Technischer Uberwachungs-Verein Bayern in Munich, West Germany, in which a scaled tread pattern is drawn on a band of paper, with the load supporting portions (lugs) shaded in black, and the grooves in white. The band is mounted on a rotatable drum having a fixed circumference. A light is shone on the pattern and reflected light is received by up to 40 photodetectors arranged side-by-side to extend across the width of the tread pattern. As is evident, this is a simple analog device to produce a pulse width modulated signal which may be correlated to the actual sound produced by the tread pattern. The pulse width is equal to the dwell time of the block as it traverses each detector or channel.
Among the limitations of such a device is that the model (pulse width signal) for the sound produced is fixed by the hardware and cannot easily be modified or fine-tuned. Also, the model is insensitive to speed changes since a change of speed simply dilates or contracts the period of the waveform. Further, a new drawing must physically be made for each modification of block and channel configurations in modified tread designs. Still further, the size and sensitivity limitations of the photodetectors are exacerbated by the necessity of having to change their physical location to lie along the profile (edge) of a footprint, assuming such a refinement was desired over simply aligning the photodetectors across the width of the pattern.
It occurred to us that the foregoing limitations could be obviated if a noise evaluation could be predicated upon that generated by a design unit of a tread. Design unit refers to an arbitrarily chosen characteristic portion of the tread which is representative in general form, but with scale variations which cumulatively compose the tread. A preferred design unit is typically bounded by the circumferential centerline of the tread, a furthermost circumferential edge in contact with the road (contact edge), and predetermined lateral spaced apart boundaries.
No prior art has considered the use of a design unit as the only critical element necessary for making the aural evaluation with the help of appropriate software.
The device of our system, being purely digital, not only avoids the limitations of the prior art analog device but lends itself to being fine-tuned for a host of refinements to mimic the actual operation of the tread on a tire at varying speeds, loads, and inflation pressures on particular road surfaces. Under actual operation, each point on the tread's surface may not be independent of neighboring points. In other words, when a point is in contact with the road surface, there may be some, though slight, interaction with the points which are immediately precedent, also with the points on either side, and the points immediately subsequent. Further, the loading on a point near the center of the tread may not be the same as that on a point near an edge, nor would the contribution of each be the same based on their relative positions. It is such refinements which may be weighted into the construction of a sound waveform using our method. It lends itself to being tailored to provide the level of sophistication demanded of it.
The optimum test for evaluating the acceptability of the noise level of a tire is to mount a set of identical tires on a specific vehicle, and run it over a designated surface at various speeds, loads, and inflation pressures in the vehicle's operating range while a person within the vehicle listens to the sound generated by the tires. Of course, to do so, one must design the tread, construct green tires, build a mold to cure the green tires, and test the cured tires on the vehicle. This invention obviates the foregoing steps, yet arrives at a reliable evaluation. This allows the design engineer to screen a virtually unlimited number of variations of a basic design by editing tread designs, much like a word processor edits test. Since the noise generating mechanism is very complex, and not well understood, it is not expected that this invention will totally eliminate the need for actual testing.
Recognizing that it is self-evident positioning of ribs relative to each other will influence the level noise, in the past, either an upper or lower portion of a tread ring has been arbitrarily displaced relative to the other portion in a tire mold. The resulting cured tire was then tested with the expectation that an improvement would result. Aside from the obvious hit or miss approach of this technique, it suffers the limitation that an entire one half of a tread pattern must be rotated relative to the other half. Our methodology provides a logical approach to reducing the tread noise.